The flame atomic absorptiometry has excellent features of being able to perform measurements easily and with high sensitivity because it has a short analysis time and is little influenced by chemical interference. It can measure as many as 80 kinds of atoms and has a wide range of applications including measurement of impurities in steels and trace metals in food, and measurement of trace metal components in environmental specimens. As a growing number of regulatory limits are being changed in an environmental analysis field these days, the role of the flame atomic absorptiometric analysis apparatus is expanding.
An example analysis method with a particularly wide range of applications is a metallic hydride generation analysis method. This method is capable of determining a trace amount of hazardous metals such as arsenic and selenium, and is a common analysis technique widely used in the management of clean water and waste water and in environmental fields. This method first transforms an objective metal in a specimen solution into a gaseous hydride by chemical reaction. The hydride is then introduced into a cell made of quartz, which is heated by a flame of the flame atomic absorptiometer. The hydride in the cell on heating is dissolved into hydrogen and a vapor of the metal being measured. This specimen metal vapor is measured by the atomic absorptiometry.
Most of the hydrides generated by chemical reactions can be measured and determined with a very high sensitivity (sub ppb) because their evaporation temperatures are low. This analysis method, however, has the drawback that because the quartz cell cannot be placed in the magnetic field, measurements need to be done by a method without a background correction (single beam measurement) or a D2 method with a degraded background correction capability.
The configuration of a conventional flame atomic absorptiometer is shown in FIG. 2. Measurement of hydrides with this absorptiometer is performed as follows.
A solution of a specimen containing metals to be measured, such as arsenic and selenium, hydrochloric acid and sodium borohydride is prepared and then delivered and mixed by a peristaltic pump 10. The mixed metals to be measured react to become a hydride. The hydride thus obtained is introduced into a gas-liquid separation unit 12 called a separator where it is separated into a specimen gas to be measured and others. The specimen gas is fed through a specimen introducing pipe 14 into a quartz cell 16, in which it is heated by a flame 20 of the flame atomic absorptiometer and evaporated into an atomic vapor. A beam emitted from a light source, for example, a hollow cathode lamp 18, is passed through the cell 16. The hollow cathode lamp 18 emits a beam 24 having a line spectrum of an element being measured. As this measuring beam penetrates through the specimen atomic vapor, light absorption takes place. The beam is then scattered by a spectroscope 22 into a spectrum which is detected by a detector 26 for atomic absorption measurement.
The drawback of this measuring method is that because the measurement is made by using a quartz cell about 16 mm in diameter, the distance between magnetic poles must be set to more than 16 mm to perform the light absorption measurement with background correction utilizing the Zeeman effect and it is difficult to realize a magnet of such a large size. Further, because the heating section and the measuring section are the same, locating the cell between the magnetic poles results in the magnet being heated, too, which makes this configuration impracticable.
For the reasons mentioned above, it is conventionally impossible to arrange the cell in the magnetic field and there is no alternative but to use a single beam method or a D2 method with a degraded background correction capability. This has given rise to various problems in measurement, such as variations in absorption intensity, caused by base line variations and wavering of flame on the optical path. It is also necessary to dismount and remount the magnet as heavy as 10 kg in performing measurement.